1. Field of the Invention
This invention relates to ultrasonic monitors and is primarily concerned with ultrasonic movement monitors such as a monitor or apparatus for monitoring breathing movement of a subject, which may be a human being or an animal. However, the invention also has utility in other fields where it is necessary to exclude certain movements and to detect other movements.
2. Description of the Prior Art
It has been heretofore proposed to monitor the breathing of an infant by monitoring ultrasonic waves reflected from the body of the infant, in order to detect the cessation of respiration. An alarm system is initiated when breathing has ceased, to call the parent or a nurse in a hospital so that immediate and urgent attention can be provided to the infant. One such system is U.S. Pat. No. 4,197,856 which issued on Apr. 15, 1980 to Robert B. Northrop entitled "Ultrasonic Respiration/Convulsion Monitoring Apparatus and Method for Its Use". The system utilizes a continuous ultrasonic wave and a phase-locked-loop is provided whereby the frequency of the ultrasonic source is varied. The analog output signal of the phase-locked-loop represents the movement of the object or person in the time domain.
U.S. Pat. No. 4,738,266 dated Apr. 19, 1988 issued to John B. Thatcher and is entitled "Apnea Monitor". The patent indicates that the exhaled breath of an infant is collected in a hood. A source of infrared energy emits infrared energy into the hood. So long as the infant is breathing, the carbon dioxide in its breath absorbs a portion of the infrared energy in the hood. Should the infant stop breathing, an infrared detector responds to the resulting increase in infrared energy to activate an alarm so as to enable the attendant personnel to take appropriate action.
U.S. Pat. No. 5,638,824 issued on Jun. 17, 1997 to Jonathon Brian Summers and is entitled "Ultrasonic Monitor". The monitor operates by periodically emitting burst of ultrasonic pulses from an emitter and subsequently detecting the pulses by a receiver, for example, after reflection from an object such as a sleeping infant. Changes in conditions are determined by monitoring the changes in phase occurring between the input to the emitter and the output from the receiver
While some of the prior art devices referred to previously utilize the same principle of synchronous detection that is in monitoring changes in the reflected signal with respect to the emitted signal, the present invention is arranged as a lock-in amplifier and processes analog signals only. This results in a possibility of measuring not only changes in the phase shift between the two signals, as provided in U.S. Pat. No. 4,197,856, U.S. Pat. No. 5,638,824 and U.S. Pat. No. 5,915,322, but also changes in the amplitude of the reflected signal, which the other devices do not utilize. This makes the overall sensitivity of the present invention higher. The additional measurement does not require any additional hardware and is done simultaneously with the measurement of the phase shift by the multiplication of the two signals with just one analog multiplier integrated circuit.